Heart failure is a condition wherein the heart fails to circulate blood sufficiently to meet the metabolic demands of the various tissues of the body. In advanced heart failure patients, cardiac output is maintained both by compensatory physiological mechanisms (such as salt and water retention, vasoconstriction, increased sympathetic activation, and cell growth), and therapy. This condition is referred to as compensated heart failure. These individuals may take various medications for their condition, may be on special diets, and may have cardiac rhythm management devices implanted within them to provide electrical therapies for their hearts (e.g., to synchronize cardiac contractions, and correct for unusually fast or slow beats). Despite these measures, patients usually undergo acute worsening of their heart failure condition during which the cardiac compensatory mechanisms are maladaptive and the patient suffers from hemodynamic abnormalities, respiratory distress, end organ failure, arrhythmias, and progressive heart failure. This condition is called acute heart failure decompensation. If the patient is not stabilized quickly, acute heart failure decompensation can be fatal. By managing their conditions carefully, patients may reduce the number of episodes in which they experience heart failure decompensation.
Multiple factors may precipitate heart failure decompensation. Some patients may deviate from their heart failure management regimen. Other times, decompensation occurs because of factors beyond the control of the patient such as pulmonary infection/embolism, atrial fibrillation, anemia, and renal insufficiency. In either event, early detection of impending decompensation of heart failure is important. If impending decompensation of heart failure is detected with sufficient warning, health care personnel may be able to examine the patient's condition prior to the event, and may be able to initiate therapies that ameliorate the patients condition. Despite the fact that multiple factors may precipitate decompensation, the predominant pathophysiological pathway leading to acute HF decompensation is salt and water retention that ultimately leads to fluid volume overload and pulmonary edema. Thus, if fluid volume overload can be detected early, health care personnel may recognize the patient's situation, and may respond, for example, by prescribing diuretics that remove fluid by increasing urine discharge. Existing methods for the detection of worsening heart failure measure events that may occur later in the cascade of fluid retention.
To allow for the possibility of delivering appropriate medical attention to a patient before the patient exhibits heart failure decompensation, it is desirable to obtain early warning. Preferably, the method by which warning is obtained is reasonably reliable, in order to reduce the number of “false alarms,” so that medical attention can be visited upon those who need it, without also visiting such attention upon a great number of patients who are not in need of immediate medical attention.